Recently, the use of hazardous metals such as lead, mercury, cadmium, and hexavalent chromium has been restricted by environmental regulations such as RoHS (Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment) and ELV (End of Life Vehicles) Regulations.
In accordance with this movement, a chromate film, which is effective as an anticorrosive chemical conversion coating for a member having a metallic surface such as a galvanized member, has been formed not by a composition for chemical conversion treatment (which may be referred to below as a chemical conversion treatment solution) using a chromate salt containing hexavalent chromium but by a chemical conversion treatment solution containing trivalent chromium. A chemical conversion coating obtained from a conventional chemical conversion treatment solution containing hexavalent chromium has hexavalent chromium which is soluble in water. Therefore, use of such a film is restricted by the above-mentioned regulations.
Although a chromate film formed by a chemical conversion treatment solution containing trivalent chromium has become common, it has been found that an extremely small amount of hexavalent chromium is detected in the chemical conversion coating formed by the chemical conversion treatment solution. Depending on the method of measurement, the amount of elution from the coating is roughly 0.1 microgram/cm2 or less, which is far smaller than the amount of elution from a chemical conversion coating obtained from a conventional chemical conversion treatment solution containing hexavalent chromium.
Nevertheless, methods of decreasing the amount of hexavalent chromium from a chemical conversion coating obtained from a conventional chemical conversion treatment solution containing trivalent chromium (which may be referred to below as a chemical conversion coating) are being investigated.
Patent Document 1 discloses a method of decreasing the amount of elution of hexavalent chromium by immersing a metallic member having a chemical conversion coating in a reduction treatment solution containing a prescribed content of a reducing agent for a prescribed period as a process for reducing hexavalent chromium, followed by drying the metallic member after immersion. L-ascorbic acid is preferably used as the reducing agent.
Patent Document 2 discloses a method of adding a reducing agent to reduce hexavalent chromium to trivalent chromium in a chemical conversion treatment solution. A preferable reducing agent for this method is sodium bisulfite, which is added before and after adding a supporting salt.
Patent Document 1: JP2005-240084A
Patent Document 2: JP2006-28547A